what this is and what size bolts I need?

First 2 pictures:

Can you please tell me what this is and what size bolts I need?
My front-end shakes at high speeds like it's on a paint mixer. My ex said he tightened two bolts, and it would be okay until they came loose again.

Last picture:
While i was trying to tighten the other bolts, I noticed this. What is this? Just to make sure I'm correct. How many bolts should it have, I only found one bolt. What size do I need to get?

Thank you for any help you can provide.